Insulin-like growth factor I binding proteins in dairy cows, calves and bulls

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Our objective was to measure relative amounts and distributions of serum IGF-I binding proteins and concentrations of hormones and metabolites in dairy cows during the dry period and during lactation, as well as in calves and in growing bulls. Concentrations of IGF-I were lower in cows during lactation than during the dry period. Concentrations of IGF-I, growth hormone, insulin, T3, T1 and glucose were higher in calves and bulls as compared with cows, whereas concentrations of non-esterified fatty acids, urea, protein and albumin were lower. Three different IGF-I binding protein fractions were found with apparent molecular weights of greater than 200, 140-160 and 45-65 kD. Relative amounts of IGF-I binding proteins were similar, although great differences in hormones and metabolites were found in cows, bulls and calves. accessible binding sites were higher in dairy cows than in calves and bulls, mainly owing to the fact that dairy cows had lower concentrations of IGF-I bound to these fractions. additionally, we found significant negative correlations between total accessible binding sites and total IGF-I concentrations. There were significant differences in the distribution of binding proteins. In particular, there was a shift of binding ability from the 140-160 to the 45-65 kD binding sites from the end of pregnancy to early lactation. Amounts of IGF-I bound to proteins increased from lactating to dry cows and calves and were highest in bulls. IGF-I levels in the greater than 200 kD fraction were lower in lactating than in dry cows, and lower in cows than in calves and bulls. IGF-I measured in the 140-160 kD fraction was lowest in lactating cows and comparable in dry cows, calves and bulls. Concentrations of IGF-I in the 45-65 kD fraction were lowest in cows and highest in bulls. In conclusion, there were marked differences in growing, pregnant and lactating cattle, particularly as concerns accessible protein binding and amounts of IGF-I bound to proteins.